The present invention relates to a magnetic eraser, in particular, relates to a magnetic bulk eraser which can erase the magnetic recording on a plurality of magnetic cards, a plurality of magnetic sheets, or a reel of magnetic tape in a short time with an alternating magnetic field.
When we want to erase a magnetic recording on a magnetic tape, or any other magnetic recording medium, the magnetic tape must be positioned in an alternating magnetic field, the strength of which is greater than the maximum magnetic flux density of the hysteresis loop, and the strength of the field is then gradually reduced. Or the magnetic tape must pass through the area where said strength of magnetic field is applied so that the tape receives a gradually reducing flux. The magnetic medium is erased by the process that the B-H curve changes from the maximum hysteresis loop to a minor loop and the residual magnetic flux density reaches zero. A magnetic tape can be erased by running the tape through the erasing magnetic field in order to apply a gradually reducing magnetic flux to the tape. But this process takes a long time. Therefore, a bulk magnetic eraser which can erase the magnetic medium without running the tape but just by putting the reel of tape in the erasing field is preferable. Accordingly, the present invention relates to the structure of a bulk eraser.
A prior magnetic bulk eraser utilized an electromagnet excited by an alternating current. When the magnetic medium to be erased is for instance a chromium oxide type magnetic medium, the coercive force of which is very large, a strong erasing flux of more than 1000 gauss is necessary. Therefore, an apparatus for generating such a strong flux is very bulky. Accordingly, a prior magnetic bulk eraser has been used only in tape manufacturing factories, tape copy factories and/or studios, but is not practical for home use.